1. Field of the Invention
The present invention relates to a broadcast receiving apparatus that receives incoming airwaves such as radio broadcasting and television broadcasting, and a method of determining the presence of noise performed by a broadcast receiving apparatus.
2. Description of the Related Art
Conventionally, a broadcast receiving apparatus incorporated in a vehicle or the like receives incoming airwaves such as radio broadcasting and television broadcasting, and acoustically outputs a broadcast signal in the received airwaves from an in-vehicle speaker (see, for example, Japanese Patent Application Laid-open No H8-274663). Further, in a broadcast receiving apparatus, noise components are mixed in a received broadcast signal because a reception environment of airwaves changes with a movement of a vehicle having the broadcast receiving apparatus incorporated therein. Therefore, in conventional broadcast receiving apparatuses, to ensure the listenable or viewable level of broadcast signals for passengers, a function of removing noise components in the broadcast signal has been widely used.
For example, a broadcast receiving apparatus receives incoming airwaves, and performs AM detection on an intermediate frequency signal (hereinafter, simply “IF signal”) of the broadcast signal in the received airwaves to acquire an AM detection signal. Next, the broadcast receiving apparatus detects AM noise components based on an AM detection level of the AM detection signal. Upon detection of the AM noise components, the broadcast receiving apparatus then determines the presence of noise in the broadcast signal based on the AM noise components. Subsequently, the broadcast receiving apparatus determines the presence of noise in the broadcast signal, and when there are any noise components, removes these noise components from the broadcast signal, and performs a blanking process for performing signal interpolation on 0a removed section, thereby removing noise components in the broadcast signal. This function has been widely known.
Further, there is another broadcast receiving apparatus that receives AM broadcast airwaves, performs FM detection on an AM broadcast signal in airwaves to acquire an FM detection signal, and extracts FM noise components from frequency fluctuation components in the FM detection signal. Further, there is also known a function of the broadcast receiving apparatus such that when a level of an extracted FM noise components is high, an output level of an AM broadcast signal is decreased, thereby alleviating an influence of noise components in the broadcast signal.
The conventional broadcast receiving apparatus presets a certain AM noise threshold Vtha, and determines whether the AM detection level of the AM detection signal is equal to or higher than the AM noise threshold Vtha. The broadcast receiving apparatus further detects a signal portion equal to or higher than the AM noise threshold Vtha of the AM detection level as an AM noise component. The broadcast receiving apparatus then determines the presence of noise in the broadcast signal corresponding to the AM detection signal, based on the AM noise component.
FIG. 14 is a schematic diagram for explaining an example of an AM detection level with respect to an IF signal. In the AM detection level shown in FIG. 14, for example, four types of noise statuses at a timing t1 to a timing t4 can be assumed. A timing t2 corresponds to a noise status where noise components are actually mixed and it is determined that the AM detection level is equal to or higher than the AM noise threshold Vtha. A timing t3 corresponds to a noise status where noise components are not actually mixed and it is determined that the AM detection level is lower than the AM noise threshold Vtha. The timing t1 corresponds to a noise status where although noise components are actually mixed, the noise component level is low, and it is determined that the AM detection level is lower than the AM noise threshold Vtha. The timing t4 corresponds to a noise status where although noise components are not actually mixed, it is determined that the AM detection level is equal to or higher than the AM noise threshold Vtha due to an influence of modulation components or the like in the AM detection signal.
However, in the conventional broadcast receiving apparatus, when the noise component level is low and it is determined that the AM detection level is lower than the AM noise threshold Vtha although noise components are actually mixed, the AM noise components cannot be detected, although there are noise components. As a result, in the conventional broadcast receiving apparatus, a broadcast signal is erroneously determined as a broadcast signal without noise, based on a detection result indicating that there is no AM noise component.
Further, in the conventional broadcast receiving apparatus, when it is determined that the AM detection level is equal to or higher than the AM noise threshold Vtha due to an influence of modulation components or the like although noise components are not actually mixed, modulation components equal to or higher than the AM noise threshold Vtha are erroneously detected as AM noise components. As a result, in the conventional broadcast receiving apparatus, a broadcast signal is erroneously determined as a broadcast signal with noise, based on the erroneously detected AM noise components.